>> pleasure. >> heather: nasa says this is once in a lifetime event. how do we know that? >> well, let me first say, of course, the biggest concern at the moment is for the situation in russia. so many windows were broken we are hoping that the damage can be recovered quickly. for a scientist it's a great opportunity to learn about this type of impact. we know that this was unusual event because it created a very loud boom. a signal at a frequency that is lower than what you can hear by the ear which travels long distance. this was picked up somewhere between 300 and 500 kiloton impacted. >> heather: how do we know it's not going to happen again? they said it's once in a lifetime event but how do we know that? >> statistically with these type of events they happen between once in every 20 to hundred years. in that sense. this is likely event you will experience in your lifetime. with that said, its statistic. we have coincidences that happen. >> heather: i have to ask you. an explosion estimated to have been powerful as 20 hiroshima bombs, estimated mass of 10,000 tons but